In recent years, ink-jet recording systems have been applied to various printing fields such as photography, various printing, marking, and special printing such as color filters due to its capability of forming images simply and easily. It has become possible to achieve image quality comparable to silver salt photography, specifically by employing recording apparatuses which eject minute dots under control, inks which exhibit an enhanced color reproduction range as well as enhanced durability, and improved ejection adaptability, and special sheets which exhibit markedly improved ink absorbability, color forming properties of colorants and acceptable surface gloss. Enhancement of image quality of the current ink-jet recording system is achievable only by simultaneously improvement of all the recording apparatuses, inks and special sheets.
However, ink-jet systems, which require special sheets, results in problems such as limitations of recording media as well as an higher cost of the aforesaid recording media. As a result, a number of trials have been made in which recording is carried out on image receiving media different from such special sheets, while employing various ink-jet systems. Specific examples include a phase change ink-jet system using a wax ink which is solid at room temperature, a solvent based ink-jet system using an ink comprised of rapid drying organic solvents as a main component, and a UV ink-jet system in which after image recording, crosslinking is carried out employing ultraviolet radiation (UV radiation).
Of these, in recent years, the UV ink-jet system has received attention due to relatively low generation of unpleasant odors, rapid drying properties, and recording capability onto recording media exhibiting no ink absorbability. Ultraviolet radiation curing ink-jet inks are disclosed, for example, in Japanese Patent Publication No. 5054667, Japanese Patent Publication Open to Public Inspection No. 6-200204, and Japanese Patent Publication Open to Public Inspection (under PCT Application) No. 2000-504778.
However, even though these inks are employed, it is very difficult to form highly detailed images on all the recording materials due to large changes of ink dot diameter after the impingement, depending on the types of recording materials and operation environments.
On the other hand, it is known (e.g., Japanese Patent Application Open to Public Inspection No. 2000-186071, pages 4–7) that sulfonium salts are incorporated in the aforesaid ultraviolet radiation curing ink-jet ink as an initiator. However, the ultraviolet radiation curable ink, employing such sulfonium salt initiators results in the problems described below.
Namely, the aforesaid ultraviolet radiation curable ink is divided mainly into a radical polymerization type ultraviolet radiation curable ink mainly comprised of an acryl based composition, and a cation polymerization type ultraviolet radiation curable ink. The radical polymerization type ultraviolet radiation curable ink exhibits problems in which from the standpoint of the polymerization mechanism, its curability is degraded due to the presence of oxygen which hinders curing. On the other hand, the cation polymerization type ultraviolet curable ink does no exhibit degradation of curability due to the presence of oxygen. However, problems occur in which curing tends to be adversely affected by water (humidity) at a molecular level from the standpoint of its polymerization reaction.